1. Field of the Invention
This invention relates to a process for producing olefin polymers. More particularly, it relates to a process for producing olefin polymers by use of a novel catalyst containing either a combination of a vanadium compound and a hafnium compound or a combination of a titanium compound, a vanadium compound and a hafnium compound.
2. Description of the Prior Art
Heretofore it is well known to polymerize .alpha.-olefins such as ethylene by the use of so-called Ziegler catalysts composed of transition metal compounds and organoaluminum compounds. For instance, in Japanese Patent Publication No. 49-11269, there has been proposed a catalytic system which comprises an eutectic crystal material obtained by reducing a mixture or reaction product of a titanium tetrahalide and a vanadium oxyalkoxide with an organoaluminum compound, and an organoaluminum compound. This catalyst system has very great advantages in production of polymer that it exhibits very high catalytic activity and that polymers obtained by the use of the catalytic system are narrow in distribution of particle size and high in bulk density.
The polymer obtained by the use of such catalytic system is narrow in molecular weight distribution and has been thus found suitable for application to injection molding, but has been found unsuitable for applications which requires relatively wide distribution of molecular weight such as in extrusion or blow molding.
In order to prepare polymers having a wide molecular weight distribution by using such catalyst which exhibits high activity for polymerization but serves to produce a polymer with a narrow molecular weight distribution, there have been proposed multi-stage polymerization methods or methods of blending polymers of different molecular weight. However, any of these methods require large-sized and complicate apparatus or need high technics of controlling a molecular weight ratio and a blending ratio of polymers to be blended.
Further, it is also known to use two or more catalytic components in combination so as to produce polymers with a wide molecular weight distribution. As an example of such combined catalyst there can be mentioned a catalyst using, in combination, catalytic components such as TiOCl.sub.2, ZrCl.sub.4 and the like compounds capable of producing relatively high molecular weight polymers and ordinarily employed catalytic components of capable of producing relatively low molecular weight polymers. We have already proposed in our Japanese Patent Application No. 53-64740 use of a combined catalyst of titanium, vanadium and zirconium compounds for a process of producing polymers with a wide molecular weight distribution. This process itself is useful but such catalyst is not satisfactory in its polymerization activity as applied for producing polymers with a very wide molecular weight distribution. In order to overcome the above disadvantage, a catalytic component serving to produce a relatively low molecular weight polymer and a catalytic component serving to produce a high molecular weight polymer should both exhibit high catalytic activities when employed to polymerize an olefin under the same conditions. Particularly, a catalyst serving to produce high molecular weight polymer which has been hitherto found to show low activity should be rendered highly active.
We have made an intensive study of such catalysts and, as a result, found that a solid catalyst containing hafnium, vanadium and a halogen exhibits high catalytic activity and ensures high molecular weight polymers. It has been already known to use a hafnium halide for polymerizing .alpha.-olefins, particularly ethylene, but such halide is usually very low in polymerization activity and is not usable in a commercial scale of polymer production. It is really surprising that use of a hafnium compound and a vanadium compound in combination results in a drastic increase of polymerization activity and ensures formation of high molecular weight polymers.
Further, we have found a process in which a combination of the vanadium-hafnium-base catalyst and a titanium-base catalyst is used to produce polymers with a very wide molecular weight distribution while ensuring high polymerization activity. This process has an advantage that the molecular weight distribution of polymer obtained by the use of the catalytic system can be readily controlled by changing ratios of the titanium compound, vanadium compound and hafnium compound contained in the catalyst.